Physical Sciences Research Highlights

September 2016

Plutonium Keeps Its Electrons Close to Home

Study shows that radioactive material acts like a salt, a nuance that could benefit nuclear detectors and reactors

Plutonium is formidably complex element that does not always act as expected. New research shows that plutonium does not share electrons when it bonds with fluoride atoms. Understanding how plutonium bonds and forms molecules offers insights for nuclear power, security, and environmental remediation. Image courtesy: PNNL Enlarge Image.

Found in nuclear fuel and nuclear
weapons, plutonium is an incredibly complex element that has far-ranging
energy, security, and environmental effects. To understand plutonium,
scientists at Pacific Northwest National Laboratory and Washington State
University delved into a plutonium compound with a relatively simple
composition: plutonium tetrafluoride (PuF4). While the formula is
simple, the four bonds proved to be more complex. The electrons stay relatively
close to each atom, creating ionic bonds -- not the expected electron-sharing
covalent bonds. Even though the plutonium and fluorine atoms are tied together in
a lattice, they act as isolated atoms and form ionic bonds.

"Bonding is one of the big
questions for plutonium and its actinide neighbors on the Periodic Chart," said
Dr. Herman Cho at PNNL, who led the research. "Answering this question is of
huge importance because plutonium's chemistry depends on how it bonds. PuF4
leans toward electrostatic attraction. This work provides a clearer picture of
why that is."

Why It Matters: Plutonium
is formidably complex because of the large cloud of electrons that
surrounds its nucleus. It doesn't always act as expected. Adding to the
complexity of the element is the limited number of institutions that can safely
handle and study the radioactive element. The team's research sheds new light
on plutonium's true nature. It could provide insights about key molecules
involved in nuclear power, national security, and environmental cleanup.

"Plutonium doesn't fit within the
simple pictures that apply to lighter elements," said Cho. "This work answers
tough questions as to why plutonium acts the way it does."

Methods: The researchers began with highly radioactive PuF4
from the long-shuttered Plutonium
Finishing Plant in Washington State. At the plant, scientists created hockey-puck
sized "buttons" of plutonium. They analyzed the plutonium using nuclear
magnetic resonance spectroscopy (NMR), which elucidates key features of
electronic structure near plutonium centers. The instruments reside in the Radiochemical
Processing Laboratory at PNNL and the Rad Annex of the
U.S. Department of Energy's EMSL, a national scientific user facility. The labs
are two of the few in the world that can perform NMR measurements on
plutonium-containing solids.

Using nuclear magnetic resonance spectroscopy in EMSL’s Rad Annex, Dr. Herman Cho and his team delved into the behavior of one of the most complex elements: plutonium. Image courtesy: PNNLEnlarge Image.

Cho and his colleagues examined the
atoms in the PuF4. Specifically, they probed the fluorine atoms around
the plutonium centers to measure the magnetic fields produced by plutonium (Pu4+),
which revealed how the electrons were distributed in the sample. They
determined that the plutonium and fluorine atoms aren't particularly generous.
Both atoms tend to hold their electrons, acting more like ions in a salt where electrostatic
forces hold the atoms together.

This research brings scientists
closer to understanding the nuances of plutonium and other actinides, other radioactive
elements near the bottom of the Periodic Table.

What's Next? Cho and his colleagues are continuing
to delve into the nuances of plutonium as well as uranium, neptunium, thorium,
and similar complex actinides to understand how these elements interact with other
atoms and groups of atoms.

Acknowledgments

Sponsors: This work was supported in part by the U.S. Department of Energy
(DOE), Office of Science, Office of Workforce Development for Teachers and
Scientists (WDTS) under the Visiting Faculty Program (VFP) (CC, RJD) and the DOE
Office of Science, Office of Basic Energy Sciences, Division of Chemical
Sciences, Geosciences and Biosciences, Heavy Element Chemistry program (SS,
BKM, and HC).

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In short...

97-character summary:
Plutonium hoards electrons when it meets other elements, insight for power,
security, and cleanup

Short summary: Extremely complex plutonium has ties to energy and security. Scientists
from Pacific Northwest National Lab and Washington State University found that
plutonium's behavior, in PuF4, can be attributed to the hoarding of
electrons by the atoms.